Inorganic pigments and process for preparing same

ABSTRACT

The present invention relates to new inorganic pigments and to a process for preparing same. More particularly, it relates to new yellowish-green pigments consisting of titanium compounds. These pigments consist of the crystalline phases rutile TiO 2 , perovskite CaCO 3  and trigonal CoTiO 3 . They contain 2 to 12% by weight of cobalt and 4 to 20% by weight of calcium based on total TiO 2  and they have a dominant wave length λ D  comprised between 530 and 560 mμ.

BACKGROUND OF THE INVENTION

As is known, the most commonly used yellowish-green pigment is the green chromic oxide Cr₂ O₃. Such pigment has the drawback of being toxic.

It is known too that it is possible to obtain from the titanium dioxide pigment of rutile structure (that it is white) pigments based on titanium compounds showing different colours by reacting said TiO₂ pigment with compounds of different elements such as, for example, Ni, Cr and Co.

For instance, a bluish-green pigment has been obtained by calcination of TiO₂ with a cobalt salt and, optionally an aluminium salt (in the latter case the Al.sub. 2 O₃ /Co molar ratio in the starting mixture shall be lower than 1).

OBJECTS OF THE INVENTION

Thus it is an object of the present invention to provide new yellowish-green pigments free from chrome.

Another object is to provide the above-said pigments in the form of titanium compounds.

A further object is to provide a process useful to obtain the above-said pigments.

GENERAL DESCRIPTION OF THE INVENTION

These and still other objects are achieved by coloured pigments based on titanium compounds, which pigments consist of the crystalline phases rutile TiO₂, perovskite CaCO₃ and trigonal CoTiO₃ ; contain from 2 to 12% by weight of cobalt and from 4 to 20% by weight of calcium referred to the total TiO₂ (i.e. the TiO₂ present both in the form of TiO₂ and in the form of CaTiO₃ and CoTiO₃) and have a dominant wave length λ_(D) comprised between 530 and 560 mμ.

The pigments according to this invention may also contain aluminium; in such amount, that the molar ratio between Al.sub. 2 O₃ and Co is generally comprised between 0.01 and 0.20. Therefore, calculated in respect of the total TiO₂ of the pigments, the Al amount generally ranges from 0.018 to 2.197% by weight, depending on the Co amount contained in the pigment.

In fact it has been found, according to this invention, that while the pigments based on titanium compounds containing Co or Co and Al 1 (with Al.sub. 2 O₃ /Co molar ratio lower than 1) exhibit a bluish-green colour, the addition of Ca to the above-said systems permits one to obtain yellowish-green products, i.e., having a λ_(D) of 530-560 mμ.

The preponderant crystal phase of the pigments according to the present invention is that of rutile TiO₂. Calcium titanate CaTiO₃ having the monoclinic crystal structure of perovskite and cobalt titanate CoTiO₃ having a trigonal crystal structure are present as well. The perovskite amount increases as the calcium amount in the pigment increases. Similarly, the CoTiO₃ phase increases as the cobalt amount increases. The pigments containing Al do not show any crystalline phase of an aluminium compound. This depends at least in part on the fact that the quantity of Al present is low. It is also supposed that at least a part of the Al has been entered into the crystalline lattice of the rutile TiO₂, modifying said lattice, according to a known phenomenom, described, e.g., in the United States Patent 3.022.186.

The pigments of this invention exhibit a roundish shape and have a diameter generally comprised between 0.5 and 3.0μ.

The colour of the pigments has been defined by their dominant wave length λ_(D) (see A. G. Hardy: Handbook of Colorimetry; Massachusetts Institute of Technology; Cambridge, Massachusetts, 1936, in particular page 11).

The λ_(D) of the obtained products covers the entire range of the yellowish-green shades, namely from 530 to 560 mμ.

Thus it is possible to obtain also products having the same λ_(D) as the commercial green chromic oxides, i.e., a λ_(D) of about 554 mμ.

The cobalt amount ranges from 2 to 12% by weight in respect of TiO₂ (we always refer to the total TiO₂, as defined previously). It is possible to use lesser amounts of cobalt, but in such case the resulting shades are too light. It is also possible to use higher amounts thereof, but it is preferable not to exceed 12% in order to limit the cost of the product. Cobalt amounts ranging from 5 to 12% are preferably employed. Under these conditions the products obtained exhibit a good colour strength comparable to that of the chromic oxide green pigments.

The calcium amount ranges from 4 to 20% by weight in respect to TiO₂, the amount preferably used ranging from 8 to 20%.

In general, the aluminum amount is such as to obtain an Al.sub. 2 O₃ /Co molar ratio comprised between 0.01 and 0.20.

As it will appear later on, the presence of aluminum permits, among other things, to prepare the pigments at on the average lower temperatures, wherefore it is easier to control the pigments' sizes.

The cobalt amount being the same, the increase of the aluminum amount, in the range from 0.01 to 0.20, causes the λ_(D) to decrease, shifting it towards bluish-green.

Aluminum amounts higher than a ratio of 0.20 are generally undesired, because they can lower the λ_(D) to values below 530 mμ. The preferred ratio is comprised between 0.10 and 0.20.

Preferred pigments are those containing from 5 to 12% by weight of Co and from 8 to 20% by weight of Ca based on TiO₂ ; such preferred pigments may also contain Al the Al.sub. 2 O₃ /Co molar ratio ranging from 0.01 to 0.20 and, more preferably, from 0.10 to 0.20.

The process according to the present invention consists in preparing a mixture of TiO₂, CoO and CaO and, in case, Al.sub. 2 O₃ or compounds of the same metals capable of yielding the above-mentioned oxides in the successive calcining operation, and in calcining the mixture so obtained, at temperatures generally ranging from 950° to 1150° C, in the presence of air.

The amounts of Co, Ca and Al compounds to be used are selected as a function of the desired pigment composition. Al; the aluminium Al.sub. Generally, the amounts employed are such that the element content, calculated in respect of the starting TiO₂, is comprised between 2 and 12% for Co, between 4 and 20% for Ca and between 0.018 and 2.197% for A1;the aluminum amount is such that the A1₂ O₃ /Co molar ratio ranges from 0.01 to 0.20.

TiO₂ or compounds capable of providing it during calcination mean in particular:

TiO₂ having a rutile or anatase structure, whose particles have, for example, a mean diameter comprised between 0.2 and 0.4μ;

The hydrated forms of TiO₂ ; particularly suited to this purpose is the gel of the meta-titanic acid TiO₂.H₂ O purified from iron, that is obtained as an intermediate product in the processes for preparing the TiO₂ pigments via titanyl sulphate (see, e.g., Kirk Othmer, Encyclopedia of Chemical Technology, II Edition, Vol. 20, pages 401-404, Interscience Publisher, New York 1969).

Besides CoO, CaO and Al₂ O₃, it is possible to use the cobalt, calcium and aluminium compounds that decompose to CoO, CaO and Al₂ O₃ at temperatures not higher than the abovesaid calcination temperatures: for example--cobalt sulphate, nitrate, acetate and sulphide; calcium nitrate and carbonate and aluminum sulphate.

The preferred source of TiO₂ is the gel of TiO₂.H₂ O.

Admixing of the starting compounds is generally effected in water, for example, at room temperature; admixing is carried on for a time period sufficient to ensure a good homogenization, such, for example, a 10 to 60 minutes mixing period.

The mixture can then be dried, for example, at 110°-130° C. The dried product is then homogenized by grinding and subjected to calcination.

When use is made of a furnace capable of ensuring a good mixing of the material to be calcined, for example, a rotary furnace, drying can be omitted. The furnace can be directly fed with the paste obtained at the conclusion of the admixing step.

Calcination is generally conducted at 950°-1150° C, in the presence of air, employing a residence time preferably comprised between 1 hour and 3 hours. Generally it is preferable to avoid working at temperatures higher than 1150° C, as in such case the pigment particles could tend to become too large.

Calcination temperatures are selected as a function of the pigments to be obtained, taking the following factors into account:

In the presence of aluminum, calcination can be carried out at lower temperatures;

Either in the absence or in the presence of aluminum, calcination can be carried out at lower temperatures when the product contains relatively high amounts of both calcium and cobalt.

Calcination temperatures are preferably selected -- as a function of the desired product -- in the ranges reported on Table A.

Thus, the preferred products, i.e., those containing from 5 to 12% of cobalt and from 8 to 20% of calcium, and optionally aluminum, are generally obtained at temperatures comprised between about 1000° C and about 1150° C.

                                      TABLE A                                      __________________________________________________________________________     Product composition                                                            Cobalt  Calcium                                                                (% b.w. in res-                                                                        (% b.w. in res-               Calcination                              pect to TiO.sub.2)                                                                     pect to TiO.sub.2)                                                                        Aluminium          temperature                              __________________________________________________________________________     2-5     4-20     absent or present in an Al.sub.2 O.sub.3 /Co                                                       about 1100 to about                                        lower than 0.10     1150° C                            >5-12   ≧4 and lower                                                                     absent or present in an Al.sub.2 O.sub.3 /Co                                                       about 1100 to about                               than 8   lower than 0.10     1150° C                            >5-12   ≧8  up to 20                                                                     absent or present in an Al.sub.2 O.sub.3 /Co                                                       about 1000 to about                                        lower than 0.10     1150° C                            2-6     4-20     present in an Al.sub.2 O.sub.3 /Co ratio of                                                        about 1000 to about                                        to 0.20             1150° C                            >6 up to                                                                               ≧4 and lower                                                                     present in an Al.sub.2 O.sub.3 /Co ratio of                                                        about 1000 to about                       12      than 12  to 0.20             1150° C                            >6 up to                                                                               ≧12 up to 20                                                                     present in an Al.sub.2 O.sub.3 /Co ratio of                                                        about 950 to about                        12               to 0.20             1150° C.                           __________________________________________________________________________

The calcined and cooled product is successively ground until, e.g., its residue on a 325 mesh screen does not exceed 0.1% by weight.

The pigments object of the present invention can be used in the fields in which the coloured inorganic pigments are commonly used, i.e., in particular, as coloured pigments for paints and plastic materials.

The following examples are given to better illustrate the inventive idea of the present invention.

SPECIFIC DESCRIPTION OF THE INVENTION EXAMPLE 1

As a TiO₂ -generating compound use was made of a gel of metatitanic acid at 32.8% of TiO₂, purified from iron, obtained as an intermediate product in the production of TiO₂ via titanyl sulphate; its impurities total content was lower than 200 ppm calculated on TiO₂ . 11.85 g of Co(NO₃)₃.6H₂ O dissolved in 20 cc of water and 6 g of CaCO₃ were added to 61 g of gel, corresponding to 20 g of anhydrous TiO₂. The paste was homogenized under stirring at room temperature and dried at about 110° C. The dry product was pulverized in a mortar; it was placed into a crucible and calcined in a muffle furnace, in the presence of air, at 1000° C. The product was kept at such temperature for 1 hour, whereupon it was cooled and pulverized in a mortar.

The product contained 11.4% by weight of cobalt and 12.0% of calcium (in respect to TiO₂).

Its colorimetric characteristics were determined on a dry paint film prepared starting from a vehicle made up of:

    ______________________________________                                         Aroplaz 1279 (commercial alkyd resin:                                            68% by weight; soybean                                                         oil: 32%           76.3  %                                                   FL 30 (boiled linseed oil)                                                                          19.0  %                                                   Drying mixture       4.7  %                                                     The drying mixture had the following composition:                             Calcium naphthenate  1.77 %                                                    Zirconium naphthenate                                                                               5.31 %                                                    Cobalt naphthenate   6.9  %                                                    White spirit         86.02 %                                                   ______________________________________                                    

On a dry paint film of 50μ thickness, containing 50% by weight of pigment (calculated on the dry paint), tristimulus values X, Y, Z were measured in respect of a standard calibrated to magnesium oxide by means of a differential tristimulus colorimeter Ducolor, mod. 220, manufactured by Neotec Instruments Corp.

The obtained values were processed according to the C.I.E. (Commission Internationale de l'Eclairage) coordinates system and the dominant wave length was determined graphically (for further details see the above-cited treatise by A. G. Hardy). The λ_(D) resulted to be of 555 mμ. A commercial pigment of green chromic oxide Cr₂ O₃, examined according to the same method, exhibited a λ_(D) of 554 mμ.

The X-ray diffractogram of the product according to this invention revealed the presence of rutile TiO₂, that represented the predominant phase, of CaTiO₃ having the perovskite structure and of trigonal CoTiO₃.

The mean size of the ground pigment particles was of about 1.0 μ.

By way of comparison, the test was repeated without adding --calcium carbonate. The product obtained, which contained 11.4% of cobalt, exhibited a λ _(D) of 495 mμ.

Thus it is clear that the presence of calcium in the pigment shifted the λ_(D) from bluish-green to yellowish-green.

EXAMPLE 2

11.85 g of Co(NO₃)₂.6H₂ 0 dissolved in 20 cc of H₂ 0, 6 g of CaCO₃ and variable amounts of a solution of Al₂ (SO₄)₃. 18H₂ O at 259 g/l were added, to 61 g of meta-titanic acid gel, as described in Example 1. More precisely, 10 cc of the aluminum sulphate solution were added in test 2a, and 20 cc in test 2b. It was operated according to the modalities of Example 1, calcining having been conducted at 1000° C. The characteristics of the resulting products are reported on Table B.

                  TABLE B                                                          ______________________________________                                              %         %         %                                                          b.w. of Co                                                                               b.w. of Ca                                                                               b.w. of Al                                                                             Al.sub.2 O.sub.3 /Co                                                                   λ.sub.D                        Test in respect                                                                               in respect                                                                               in respect                                                                             molar   in                                    No.  to TiO.sub.2                                                                             to TiO.sub.2                                                                             to TiO.sub.2                                                                           ratio   m μ                                ______________________________________                                         2a   11.4      12        1.05    0.1      552                                  2b   11.4      12        2.10    0.2      545                                  ______________________________________                                    

The x-ray diffractometric analysis revealed the presence of rutile TiO₂, of perovskite CaTiO₃ and of trigonal CoTiO₃.

From Table B it can be inferred that the value of λ_(D) lowers as the Al amount increases.

EXAMPLES 3 TO 13

The modalities of Example 1 were followed, but varying the calcium and aluminum amounts (while the cobalt amount remained unchanged) and the calcining temperatures.

By way of comparison, tests 3 and 4 were carried out without calcium.

                  TABLE C                                                          ______________________________________                                              %       %       %                                                              of Co   of Ca   of Al                                                          in res- in res- in res-                                                                              Molar                                               Test pect to pect to pect to                                                                              ratio   Calcining                                                                              λD                           No.  TiO.sub.2                                                                              TiO.sub.2                                                                              TiO.sub.2                                                                            Al.sub.2 O.sub.3 Co                                                                    temp.   in mμ                            ______________________________________                                         3    11,4    --      --     --     1000° C                                                                         495                                 4    11,4    --      --    --      1100° C                                                                         498                                 5    11,4     4      --    --      1000° C                                                                         536                                 6    11,4     4      --    --      1100° C                                                                         554                                 7    11,4     4      2,10  0,2     1000° C                                                                         536                                 8    11,4     8      --    --      1000° C                                                                         555                                 9    11,4    20      --    --      1000° C                                                                         534                                 10   11,4    20      --    --      1100° C                                                                         554                                 11   11,4    20      2,10  0,2     1000° C                                                                         551                                 12   11,4    20      2,10  0,2     1100° C                                                                         551                                 13   11,4    20      2,10  0,2      950° C                                                                         551                                 ______________________________________                                    

By comparing tests 3-4 with test 5-6 one may notice that the addition of calcium to the TiO₂ -Co system shifts the wave length from bluish-green to yellowish-green. By comparing test 5 with test 6 and test 9 with test 10 one may notice that the rise of the calcining temperature causes the λ_(D) to increase. By comparing tests 9-10 with tests 11-13 one may notice that in the presence of aluminum it is possible to calcine at lower temperatures.

The product of test 8 exhibited a particle's mean diameter of 0.8 μ.

EXAMPLES 14 to 30

It was operated according to the modalities of Example 1, but varying the amounts of Co, Ca and Al and the calcining temperatures.

These temperatures and the characteristics of the resulting products are reported on Table D.

    ______________________________________                                         Test 21          1.0 μ                                                      Test 22          1.3 μ                                                      Test 23          1.0 μ                                                      Test 30          0.5 μ                                                      ______________________________________                                    

                  TABLE D                                                          ______________________________________                                              %       %       %                                                              of Co   of Ca   of Al                                                          in res- in res- in res-                                                                              Al.sub.2 O.sub.3 /Co                                                                   Calcining                                                                              λD                           Test pect to pect to pect to                                                                              molar   temper- in                                  No.  TiO.sub.2                                                                              TiO.sub.2                                                                              TiO.sub.2                                                                            ratio   ature   m μ                              ______________________________________                                         14   2        4      --    --      1100° C                                                                         548                                 15   2       20      --    --      1100° C                                                                         558                                 16   2        4      0,36  0,2     1000° C                                                                         552                                 17   2       20      0,36  0,2     1000° C                                                                         559                                 18   4        4      --    --      1100° C                                                                         552                                 19   4       20      --    --      1100° C                                                                         555                                 20   6        4      --    --      1100° C                                                                         553                                 21   6       12      --    --      1000° C                                                                         549                                 22   6       12      --    --      1100° C                                                                         557                                 23   6       20      --    --      1000° C                                                                         558                                 24   6        4      1,08  0,2     1000° C                                                                         540                                 25   6       12      0,54  0,1     1000° C                                                                         556                                 26   6       12      0,54  0,1     1100° C                                                                         556                                 27   6       12      0,54  0,1      950° C                                                                         556                                 28   6       12      1,08  0,2     1000° C                                                                         550                                 29   6       12      1,08  0,2     1100° C                                                                         553                                 30   6       12      1,08  0,2      950° C                                                                         550                                 ______________________________________                                     

What we claim is:
 1. A coloured pigment based on titanium compounds characterized in that it consists of the crystalline phases rutile TiO₂, perovskite CaTiO₃ and trigonal CoTiO₃ ; it contains 2 to 12% by weight of cobalt and 4 to 20% by weight of calcium, based on the weight of the total TiO₂, and it has a dominant wave length λ_(D) comprised between 530 and 560 mμ.
 2. A coloured pigment according to claim 1, characterized in that it contains also aluminium in such amount that the Al₂ O₃ /Co molar ratio is comprised between 0.01 and 0.20.
 3. A coloured pigment according to claim 1, characterized in that the cobalt amount ranges from 5 to 12% by weight, based on the weight of the total TiO₂.
 4. A coloured pigment according to claim 1, characterized in that the calcium amount ranges from 8 to 20% by weight, based on the weight of the total TiO₂.
 5. A coloured pigment according to claim 2, characterized in that the Al₂ O₃ /Co molar ratio ranges from 0.10 to 0.20.
 6. A coloured pigment according to claim 2, characterized in that the cobalt amount is comprised between 5 and 12% by weight, based on the weight of the total TiO₂, and the calcium amount between 8 and 20% by weight, based on the weight of the total TiO₂, the Al₂ O₃ /Co molar ratio being comprised between 0.10 and 0.20. 